In space applications, it is necessary to be able to orient certain elements of a satellite or of a space vehicle, such as an antenna, a mast, etc. in a predetermined direction in order, for example, to direct them toward a fixed star, to maintain a heading toward a point on the surface of the earth, or else to scan a particular zone on the surface of the earth or of a star. These positionings must be frequently corrected to compensate for inevitable drifts in maintaining this direction and these multiple corrections cause the implementation of a large number of microrotation cycles for these space appendages and for their guidance devices.
Because of the irreversible character of placing a satellite in orbit, it is necessary, during the design of a spacecraft, to allow for a very long service life, measured in number of cycles, for these items of equipment, in order to ensure that they are able to withstand these highly numerous stresses.
This phase, called fine error sensing, usually follows a deployment phase, during which the space appendages are taken out of the packaging that has been defined for the launch and are placed in position to perform their function. Even if, in some cases, it may be necessary to use multiple deployments, the latter are still very limited in number (at most a few tens, even a hundred or so) and do not require the error sensing devices to be designed to such demanding standards as for fine error sensing movements.
Since these items of equipment must operate in the space vacuum, users come up against a particular problem associated with the difficulty of ensuring good lubrication of the portions in contact during these rotations.
The lubrication of the parts making linear movements is correctly resolved by the installation of fixed metal seals and bellows enclosing the range of movement of the parts, which makes it possible to operate in a lubricated environment (by oil bath in a closed enclosure) and to allow large numbers of linear movement cycles.
This configuration cannot be reproduced for rotary movements, the seal between the guidance device and the shaft that it drives being able to be achieved only by seals and hence by friction between the rotating parts. The resultant service life for such systems is thereby greatly degraded.
Former realizations have therefore been obliged to choose, for these rotary movements, between systems based on antifriction bearings, shaft bearings, ball-and-socket joints, or roller tracks, etc. capable of ensuring angular movements of large amplitude, and systems based on flexible components, with no contact between the rotating parts, but that allow only limited angular movements.
Because of imperfect lubrication, the first are sensitive to the phenomena of seizing by molecular cohesion of the facing materials and do not have a long service life. The phenomenon is further accentuated when it is desired to make them make micromovements, because of phenomena of migration or evaporation of the lubricants. The second do not need lubrication and therefore have a service life compatible with the intended use but they are limited in their field of use by their small angular movement.